Raku

Raku is an ancient Japanese firing technique for ceramics. After the pieces are bisque fired, they are glazed with special raku glazes and fired again. The pieces are taken out of the kiln while still red-hot, and put into a barrel of sawdust. Once the sawdust catches fire from the heat of the clay, the barrel is closed and the pieces stay in the smoke. Rapid cooling causes the glaze to show hairline cracks or crackles. The smoke soaks into the hairline cracks and creates the typical raku effect. You can read more about raku here.